If you are attempting to play an MP4 video on an Android device and you hear sound, but see no picture, then it is likely due to an incompatible codec used for the encoding of your MP4 video. With the Android OS there are varying levels of support for different codecs and while it may work on one version of the OS, that doesn't mean it will always work on others.

One solution that often works is to use a video editor like HandBrake and then encode it using the default codecs for Android. These are usually set to a lowest common denominator and will often resolve your problem.